1. Field of the Invention
The invention relates to an air-fuel ratio imbalance determination apparatus and an air-fuel ratio imbalance determination method that determine an inter-cylinder air-fuel ratio imbalance state in a multi-cylinder internal combustion engine.
2. Description of Related Art
In an exhaust system of an internal combustion engine (hereinafter also referred to as an engine) mounted on a vehicle or the like, a catalyst for purifying exhaust gas (e.g., a three-way catalyst) is provided. The catalyst is capable of purifying (converting) exhaust gas components most efficiently when an air-fuel ratio of exhaust gas flowing into the catalyst falls within a predetermined range. Accordingly, an air-fuel ratio sensor is disposed in an exhaust passage at a position upstream of the catalyst, and the amount of fuel injected from an injector is controlled through feedback based on a deviation between the air-fuel ratio detected by the air-fuel ratio sensor (the air-fuel ratio of the exhaust gas flowing into the catalyst) and a target air-fuel ratio (e.g., a stoichiometric air-fuel ratio) (main feedback control (main F/B control)). By executing the air-fuel ratio feedback control described above, it is possible to control the air-fuel ratio with high accuracy and achieve an improvement in exhaust gas emission.
In addition, what is called a sub feedback control (sub F/B control) is generally executed. In the sub feedback control, the air-fuel ratio of the exhaust gas having passed through the catalyst is detected based on the output value of an O2 sensor (oxygen sensor) provided downstream of the catalyst, and the output of the above air-fuel ratio sensor is corrected.
In a multi-cylinder internal combustion engine including a plurality of cylinders, there are cases where an actual air-fuel ratio varies among the cylinders (an air-fuel ratio imbalance) resulting from a variation in the injection performance of injectors provided in the individual cylinders or a variation in intake air distribution amount among the cylinders and, when the above situation occurs, there are cases where an emission is deteriorated due to a deterioration of combustion of a specific cylinder.
To cope with this, there is adopted a method in which the inter-cylinder air-fuel ratio imbalance is suppressed by determining whether or not the air-fuel ratio imbalance state occurs based on an output signal of the air-fuel ratio sensor and correcting the fuel injection amount in a case where the air-fuel ratio imbalance state occurs (see, e.g., Japanese Patent Application Publication No. 2005-133714 (JP-2005-133714 A)). An example of a method for determining the air-fuel ratio imbalance state includes a method in which an amount of change per unit time in the air-fuel ratio detected by the air-fuel ratio sensor (hereinafter also referred to as an air-fuel ratio gradient) is detected, and it is determined that the air-fuel ratio imbalance state occurs when the air-fuel ratio gradient (an absolute value) is larger than an imbalance determination threshold value (see, e.g., Japanese Patent Application Publication No. 2011-144785 (JP-2011-144785 A)).
In the air-fuel ratio sensor disposed in the exhaust passage of the engine, the sensor output (the detected air-fuel ratio) may be a value richer than the actual air-fuel ratio due to selective diffusion of H2 in the exhaust gas. As an air-fuel ratio sensor capable of preventing this rich output, there is an air-fuel ratio sensor having a catalyst layer provided in a sensor element (hereinafter also referred to as an air-fuel ratio sensor with a catalyst layer) (see, e.g., Japanese Patent Application Publication No. 2009-075012 (JP-2009-075012 A) and WO 2010/064331).
In the air-fuel ratio sensor with the catalyst layer, it is possible to enhance the accuracy of detection of the air-fuel ratio by oxidizing (purifying) H2 contained in the exhaust gas in the catalyst layer. However, an exhaust gas component reaches the exhaust side of the sensor after being reacted and diffused in the catalyst layer, and hence the response of the sensor output is delayed. When such a response delay is caused, the above air-fuel ratio gradient used for the determination regarding the air-fuel ratio imbalance state is reduced. In order to prevent the response delay, the target air-fuel ratio is set to be rich (an air-fuel ratio enrichment control is performed), the response delay due to the catalyst layer is thereby eliminated, and the above air-fuel ratio gradient is thereby increased. However, in a case where the degree of richness provided by the air-fuel ratio enrichment control is insufficient, an estimated imbalance ratio estimated from the above air-fuel gradient (a learned imbalance value) becomes smaller than the actual imbalance ratio.